EW: Does birth order really have some power over sister relationships?

DT: Birth order is fascinating, and it is forever . I love this quote from the musical by two sisters, Bessie and Sadie Delany, called The Delany Sisters . Bessie said about Sadie, "Sadie doesn't approve of me sometimes; she kind of looks at me in that big sister sort of way." She was 101. Her sister was 103! Isn't that great—101 but still her sister is two years older, and that is always there.

EW: You interviewed so many different women for your book. What were some of the common things you discovered about sister relationships?

DT: I certainly discovered that there is a huge range of sister relationships. I was also quite struck by how almost everybody I spoke to spent a lot of time telling me how she and her sister are different. Sometimes it was, "We never got along; we are very different." And sometimes it was, "We are really different, but we are still really close." That made me wonder what is it about the question "How are we different?" why is that so central?

Thinking about it, I got a lot of perspective on the relationships. For example, you almost don't know who you are without asking who she is. It is like your sister is you and not you. Also, families tend to define each child in terms of the others: the outgoing one and the shy one, the athlete and the bookworm. And, unfortunately, we hear this one a lot: the pretty one and the smart one. Two of the cases where I encountered that, [the sisters] were identical twins, which really makes you see that a lot of it has to do with an impulse to compare and see difference in duality.

When you see things next to each other, you tend to compare. If I put two bottles of soda next to each other in front of you, you would compare them. Are they the same, or are they different? How are they the same, how are they different? I think that happens with siblings. In telling me about themselves and their relationships with their sisters, many women just started by telling me how they are different, and that, to me, was very interesting.

EW: Let's talk about competition. Why is that such a central theme with so many sisters?

DT: Like anything else, there is a range of how prominent [competition is between sisters]. There were sisters for whom the competition seemed overwhelming, and there were sisters for whom told me they had no competition. Again, you are competing for the same approval from the same parents. I think of a pair of sisters—these are women in their 60s—their mother was in her 80s, so one of the sisters said, "I spoke to mom twice yesterday." The other sister said, "I spoke to her once, but we had a better conversation!"

EW: So what you are telling me is sister relationships don't change too much as we age?

DT: They definitely evolve and change, but some of the elements I think are always there.